Chanter is one of my favorite favorite classes to play. I tend to lean toward builds that focus on shorter phrases and, therefore, utilize more invocations. However, it feels like after the 2.0 patch this build become far less useful... especially of higher levels. And this includes almost all but the more difficult battles in PoTD and Trial of Iron. The pace of combat has increased in such a way that they don't feel as much fun to play.
The best solutions I can think looks at chanters in a way similar to the other spell-caster classes. The simple solution is to treat them like wizards. All invocations cost one less phrase across the board once they get to level X. Wizards get level 1, 2 and 3 per-encounter spells at level 9, 11 and 13 respectively. Plus, they get access to all of their other spells. It doesn't seem so outrageous to scale chanters so that their invocations cost one less phrase at levels 9, 11 and 13. By the time chanters reach level 13 their level one invocations will cost one phrase. That is pretty similar to the 5-second delay that the "multiclass" talent variants of Whispers of Treason and Skeleton Summons get already. It could even be Levels 10, 12 and 14 if that makes any difference. Or it can only occur twice at levels 11 and 13 only. Whichever works best for game balance. Or, if that seems too overpowered, the chanters can always be treated like ciphers. Specifically, force builds of this manner to take a class-specific talents in order for invocations to be cast with fewer phrases. At least people will have to sacrifice certain talents with their builds in order to utilize invocations more often. Ciphers have a similar catch with regards to the fact that they have to pick up talents that maximize their focus. If simply allowing chanters to cast invocations with fewer phrases accumulated once they hit level X seems too overpowered, this seems like a solution that would make for a more balanced invocation build.